This invention relates to the preparation of inorganic-oxometalate anion pillared clay compositions having the hydrotalcite type layered double hydroxide crystal structure and more particularly, anionic magnesium-aluminum and zinc-aluminum hydrotalcite clays containing large inorganic polyoxometalate anions (POMs) with Keggin-type structures intercalated between positively charged layers of metal hydroxides.
In recent years inorganic materials such as pillared smectite clays have been extensively used as catalytic materials in varying applications. These materials comprise negatively charged metal silicate sheets intercalated or pillared with hydrated cations. See Pinnavaia T. J., Science, 220, 365 (1983) for a review on these clays. The techniques for the intercalation of these clays are well-established and wide variety of cations may be incorporated into clays such as montmorillonite. Through the changes in the size of the pillar used to separate the sheets in the clay structure, the pore size of the pillared clay may be tailored to a particular application. Porous clay materials with high surface area have been prepared using organic or organometallic cations, metal chelates, polyoxometalate cations and transition metal halide clusters. Synthesis of several such systems has been disclosed in several patent literatures including the ones by Pinnavaia et. al., in U.S. Pat. Nos. 4,665,045; 4,665,044 and 4,621,070.
Polyoxometalate anions are another class of pillars that are suitable for lamellar solids. POMs containing early transition metals form water soluble anions with the general formula [M.sub.m O.sub.y ].sup.p- (isopolyanions) and [X.sub.x M.sub.m O.sub.y ].sup.q- (x&lt;m) (heteropolyanions). M is usually molybdenum or tungsten, less frequently vanadium, niobium or tantalum, or mixtures of these elements, in their highest oxidation states. For general review on polyoxometalate anions see, Pope, M. P., Heteropoly and Isopoly Oxometalates, Springer-Verlag, New York, (1983). POMs forms a structurally distinct class of complexes based predominantly, although not exclusively, upon quasi-octahedral-coordinated metal atoms (MO.sub.6).
The simplest POMs have the hexametalate structure M.sub.6 O.sub.19, where the oxygen atoms are in closed-packed arrangement with six MO.sub.6 octahedra (FIG. 1A). Some of the isopoly anions include [Nb.sub.6 O.sub.19 ].sup.8-, [Ta.sub.6 O.sub.19 ].sup.8-, [Mo.sub.6 O.sub.19 ].sup.2- etc. The decavanadate anion V.sub.10 O.sub.28.sup.6- has a related structure (FIG. 1B). Similarly, seven edge-shared octahedra form the Anderson-type structures (FIG. 1C) such as in [Mo.sub.2 O.sub.24 ].sup.6-.
The most extensively studied POM compounds are those with Keggintype structures (FIG. 2). At least two isomers of the Keggin structure are known and FIG. 2A represents the .alpha.-form. The structure has overall T.sub.d symmetry and is based on a central XO.sub.4 tetrahedron surrounded by twelve MO.sub.6 octahedra arranged in four groups of three edge shared octahedra, M.sub.3 O.sub.13. These groups ("M.sub.3 triplets") are linked by several corners to each other and to the central XO.sub.4 tetrahedron. Most of these types of Keggin ions are either molybdates or tungstates with the general formula [XM.sub.12 O.sub.40 ].sup.n- where M is Mo or W. For M=W, anions with X=H, B, Al, Ga(III), Si, Ge(IV), P(V), As(V), V(V), Cr(III), Fe(III), Co(III), Co(II), Cu(II), Cu(I), or Zn have been reported. Similarly for M=Mo anions with X=Si, Ge(IV), P(V), As(V), V(V), Ti(V), Zr(IV), In (III) is known. The second isomer has the .beta.-Keggin structure, where one of the edge-shared M.sub.3 O.sub.13 triplets of the .alpha.-structure rotated by 60.degree. around the C.sub.3 axis, thereby reduction of overall symmetry of the anion from T.sub.d to C.sub.3v (FIG. 2B). This structure is known for several tungstates (X=B, Si, Ge, H.sub.2) and molybdates (X=Si, Ge, P, As).
Another class of POM are known in which a single MO.sub.6 octahedron is deficient (FIG. 2C). These are known as lacunary (defect) Keggin POM anions and has the general structures such as [XW.sub.11 O.sub.39 ].sup.n- (represents as XM.sub.11) where X=P, As, Si, Ge, B, Al, Ga, Fe(III), Co(III), Co(II), Zn, H.sub.2, Sb(III), Bi(III); or [XMo.sub.11 O.sub.39 ].sup.n- where X=P, As, Si, Ge. These anions are stable in aqueous solutions and can be isolated in pure forms. Removal of a trigonal group of three adjacent MO.sub.6 octahedra from the Keggin structure derive an another class of lacunary structures, which lead to XM.sub.9 structure (FIG. 2D). The anion [PW.sub.9 O.sub.34 ].sup.9- represents one such example. The POM [P.sub.2 W.sub.18 O.sub.62 ].sup.6- consists of two PW.sub.9 lacunary units fused into a cluster of virtual D.sub.3h symmetry (FIG. 2E). This unit is now known as the Dawson structure, and has two types of W atoms, six "polar" and twelve "equatorial". Removal of one MO.sub.6 octahedra from this Dawson structure results in [X.sub.2 W.sub.17 O.sub.61 ].sup.10- type lacunary anions.
There are several other POMs which are closely related to Keggin type structure. For example, the anion PV.sub.14 O.sub.42.sup.9- has a "bicapped Keggin" structure. In this POM twelve vanadium atoms form the usual Keggin structure and the remaining two V atoms occupy the pits on the Keggin molecule where a C.sub.4 axis is passing, forming trigonal bipyramidal caps (FIG. 2F). The POM anion NaP.sub.5 W.sub.3 O.sub.110.sup.14- has an approximate D.sub.5h symmetry and consists of a cyclic assembly of five PW.sub.6 O.sub.22 units, each derived from the Keggin anion, [PW.sub.12 O.sub.40 ].sup.3-, by removal of two sets of three corner-shared WO.sub.6 octahedra which leads to the PW.sub.6 moeity of the P.sub.2 W.sub.18 (Dawson) anion. The sodium ion is located within the polyanion on the five fold axis and is 1.25 .ANG. above the pseudomirror plane that contain the five phosphorous atoms.
The structures of heteropoly and isopoly oxometalates are not confined to the structure-types described above. There are several other variations. For example, the paratungstate anion [H.sub.2 W.sub.12 O.sub.42 ].sup.10- has a different arrangement of its twelve MO.sub.6 octahedra than in a typical Keggin-type anion. Here, MO.sub.6 octahedra are arranged in four groups of three edge-shared octahedra to form a central cavity. It has been suggested that the two protons are attached to the oxygen atoms inside the cavity and help stabilize the somewhat open structure by hydrogen-bonding (FIG. 1D).
Anionic POM compounds have been extensively used as heterogeneous catalysts for a broad variety of reactions. Examples include: oxidation of propylene and isobutylene to acrylic and methacrylic acids, ammoxidation of acrylonitrile; oxidation of aromatic hydrocarbons; olefin polymerization and epoxidation, and hydrodesulfurization. Thus negatively charged polyoxometalates would present a wider range of thermally stable, catalytically active pillars, provided a suitable host clay material is utilized.
Layered double hydroxides (LDHs), which are also referred to as anionic clays, represent a potentially important class of lamellar ionic solids for forming pillared derivatives with anionic POMs. These clays have positively charged layers of metal hydroxides between which are located anions and some water molecules. Most common LDHs are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These clays have structures similar to brucite (Mg(OH).sub.2) in which the magnesium ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In the LDHs, some of the magnesium is isomorphously replaced by a trivalent ion, say Al.sup.3+. The Mg.sup.2+, Al.sup.3+, OH.sup.- layers are then positively charged necessitating charge balancing by insertion of anions between the layers. One such clay is hydrotalcite in which the carbonate ion is the interstitial anion, and has the idealized unit cell formula [Mg.sub.6 Al.sub.2 (OH).sub.16 ]CO.sub.3.4H.sub.2 O. However, the ratio of Mg/Al in hydrotalcite can vary between 1.7 and 4 and various other divalent and trivalent ions may be substituted for Mg and Al.
The preparation of LDHs is described in many prior art publications, particular reference being made to following review journal articles by S. L. Suib et. al., in Solid State Ionics, 26 (1988), 77 and W. T. Reichel in CHEMITECH, 58 (1986). An important aspect of the synthesis of these materials, which is particularly relevant to this disclosure, is the variation of the nature of the interstitial anion. The preparation of hydrotalcite-like materials with anions other than carbonate in pure form requires special procedures, because LDH incorporates carbonate in preference to other anions. Most of the time the smaller anions are introduced to the LDH structure via the precipitation method by using the desired anion solutions instead of carbonate. In this manner the carbonate anion in the hydrotalcite can be varied in synthesis by a large number of smaller anions such as NO.sub.3.sup.-, Cl.sup.-, OH.sup.-, SO.sub.4.sup.2- etc. However, in these methods the synthesis has to be carried out in an anaerobic condition to prevent carbonate contamination from the atmospheric carbon dioxide. Miyata et. al. in U.S. Pat. Nos. 3,796,792, 3,879,523, and 3,879,525 describes hydrotalcite-like derivatives with anionic substitution including the smaller transition metal anions like CrO.sub.4.sup.2-, MoO.sub.4.sup.2-, and Mo.sub.2 O.sub.7.sup.2-.
The incorporation of larger anions in to LDH galleries, such as transition metal polyoxoanions, is not easy. This requires ion-exchange techniques subsequent to the LDH synthesis. The work by Miata et al., in Clays and Clay Minerals, 31, 305 (1983) indicated that the order of ion exchange capability of the gallery anions in hydrotalcite-like derivatives to be OH.sup.- &lt;F.sup.- &lt;Cl.sup.- &lt;Br.sup.- &lt;NO.sub.3.sup.- &lt;l.sup.- and for divalent anions, CO.sub.3.sup.2- &lt;SO.sub.4.sup.2-. Monovalent anions can be easily replaced by di- or poly-valent anions. Using this strategy, Pinnavaia and Kwon in J. Am. Chem. Soc., 110, 3653 (1988) have demonstrated the pillaring of several polyoxometales including V.sub.10 O.sub.28.sup.6- into the hydrotalcite structure containing Zn and Al metal ions in the layers. This pillared hydrotalcite-like material catalyzes the photooxidation of isopropanol to acetone.
U.S. Pat. No. 4,454,244 by Woltermann discloses the preparation of several polyoxometalate-LDH reaction products. However, no XRD or analytical data were given to support his assumption that the POMs were intercalated into the galleries of a crystalline LDH host. During the course of this work, we reproduced some of the synthetic procedures disclosed by Wolterman and found the products to be largely amorphous and impure (see the description of the invention below).
Recently, in U.S. Pat. No. 4,774,212 by Drezdon, the preparation of several Mg/Al hydrotalcite-like materials with gallery height about 12 .ANG. containing transition metal polyoxoanions such as V.sub.10 O.sub.26.sup.6-, Mo.sub.7 O.sub.24.sup.6- and W.sub.7 O.sub.24.sup.6- with Anderson type structures (FIG. 1), is disclosed.
We disclose here processes for preparing Mg/Al and Zn/Al LDHs intercalated with transition metal containing large POM anions. The products isolated were pure and gave well-defined XRD peaks corresponding to uniformly crystalline layered double hydroxide products. The basal spacings of these materials agreed with POM-intercalated structures. The replacement of smaller anions, in hydrotalcite-like LDHs with much larger POMs, particularly with POMs with Keggin type structures, produce structures with increased gallery spacings of 14 .ANG. or more. Such materials should exhibit intracrystalline microporosity accessible for the adsorption or diffusion of molecules into the structure from the outside. The accessibility of the intracrystalline structure to guest molecules should provide new opportunities for molecular seiving of gas mixtures and for heterogeneous catalysis.